Up to the present, for insulated containers which can be used as vacuum flasks, temperature maintaining lunch boxes, and the like, development of insulated synthetic resin containers, which have the advantages of being light weight, easily molded and for which manufacturing costs are low, have been progressing. As this type of insulated synthetic resin container, a container having a lid which can be fitted thereto having the following structure has been proposed. This container is an insulated container formed from an inner container and an outer container which are formed from synthetic resin. The inner container and the outer container which is dimensionally larger than the inner container are put together leaving a space therebetween, and the inner and outer containers are then joined at the respective edges of their mouth parts to give a double walled container. Then a gas having low thermal conductivity comprising at least one gas from the group consisting of xenon, krypton, and argon is enclosed in the space formed between the inner and outer containers. In the same way, the lid is an insulated lid formed from a bottom surface wall and a top surface wall which are formed from synthetic resin. The bottom surface wall and the top surface wall, which is dimensionally larger than the bottom surface wall, are put together leaving a space therebetween, and the top and bottom surface walls are then joined at the respective edges of their mouth parts to give a double walled lid. Then a gas having low thermal conductivity comprising at least one gas from the group consisting of xenon, krypton, and argon is enclosed in the space formed between the top and bottom surface wall.
After the joining of the inner and outer containers to form a double walled container, the above-mentioned insulated synthetic resin container requires a process of vacuum evacuation of the space formed between these inner and outer containers and a subsequent process of charging this space with gas, therefore, for this purpose, a penetrating hole (hereinafter referred to as a penetrating hole) which is an evacuation hole and a gas charging hole is provided in either of the inner container or outer container.
In the same way, after the joining of the top and bottom surface walls to form a double walled lid, the above-mentioned insulated synthetic resin lid requires a process of vacuum evacuation of the space formed between these top and bottom surface walls and a subsequent process of charging this space with gas, therefore, for this purpose, a penetrating hole which is an evacuation hole and a gas charging hole is provided in either of the top surface wall or the bottom surface wall.
When vacuum evacuating the contents of the insulating layer between the inner and outer containers or the top and bottom surfaces having the above-mentioned structure, the space of the insulating layer becomes gradually narrower due to deformation of the inner container and the outer container, and the top surface wall and the bottom surface wall bear the load of atmospheric pressure due to a reduction in pressure within the insulating layer. For this reason, a problem arises in which the surfaces on the insulating layer side (in other words, the outer surface of the inner container and the inner surface of the outer container, or the upper surface of the bottom surface wall and the lower surface of the top surface wall) touch and the penetrating hole for gas replacement becomes stopped up. When the penetrating hole becomes stopped up, then problems such as the following also arise.
First, during the evacuation process, resistance to evacuation becomes large and a long period of time is necessary to obtain a sufficient vacuum. In addition, situations in which the penetrating hole becomes stopped up are not all the same, so it becomes difficult to establish an evacuation time period. If the value for the evacuation time period is too short, the vacuum is insufficient, and it is not possible to introduce sufficient gas having low thermal conductivity after the vacuum evacuation.
In addition, even when vacuum evacuation is achieved, during the subsequent gas charging process, the stopped up hole results in resistance to the introduction of gas, the introduction of the gas having low thermal conductivity cannot be completed in the established time period, and because it is returned to atmospheric pressure while still in a low pressure condition, it is not possible to introduce sufficient gas having low thermal conductivity.
Furthermore, when injection molding containers which have a symmetric structure, such as kitchen and table ware, a gate is generally provided in the center of the lower surface so that the properties of injection molding will be good, weld generation will be low, and so that eccentricity in shape after the molding process can be reduced as much as possible.
However, for a double walled structure, in a vacuum evacuation process like the one mentioned above, due to the effects of atmospheric pressure, the weld mark (in particular, a pin point gate) which is in the center of the bottom surface of the inner container becomes inserted into the gas replacement penetrating hole which is in the center of the bottom of the outer container, the hole becomes stopped up and vacuum evacuation becomes impossible. For this reason, the use of a pingate in molding a container is limited, and there are problems with automation of the molding and stabilization of the cycle time.
In order to avoid this type of deformation caused by vacuum evacuation, up to the present, it has been necessary to mold the walls of containers and lids thickly. For example, when the material used is polycarbonate, a thickness of approximately 3 mm is necessary. However, when the walls are made thick, costs increase and, furthermore, it is necessary to make the space for the insulating layer thinner by the amount that the walls are made thicker, or it is necessary to make the shape of the inner and outer containers or the top and bottom surface walls larger. When the insulating layer is made thinner, the effective insulation performance of maintaining high or low temperatures is reduced. In addition, when the containers are made larger, efficient utility of space is reduced, and a problem arises in the promotion of low holding capacity increases the required space in the storage place.